1. Field of the Invention The present application relates to analogue-to-digital converters having controlled oscillators and to apparatus and methods for regulating an analogue-to-digital converter to account for, or provide, a variation in oscillator transfer characteristic.
2. Description of the Related Art
Analogue-to-Digital converters (ADCs) using voltage controlled oscillators (VCOs) are known. FIG. 1 shows an example of a conventional ADC for differential input signals that produces a digital output based on the difference between two analogue input signals. The differential analogue input signals, SP and SN, are received by respective VCOs 101 and 102 respectively. Each VCO 101, 102 outputs a steam of pulses at a pulse rate or frequency which is dependent on the input voltage, i.e. the respective input signal SP or SN. First and second pulse streams, at frequencies fP and fN respectively are thus produced by VCOs 101 and 102. The two pulse streams are supplied to difference circuitry 103 which determined the difference in the pulse rates or frequencies fP, fN. The difference circuitry comprises first and second counters 104 and 105 which count the number of pulses in a period defined by a clock signal CK to produce digital count values DP and DN respectively. These digital count values DP, DN are subtracted 106 to give the digital output value Dout.
The counters 104 and 105 may either be periodically sampled and reset, in which case the difference in their sampled counts, Dout, is then a direct digital representation of the difference between the input signals SP, SN. Alternatively, for some applications, such as the integrator of a sigma-delta ADC, the counters may run continuously, in which case the difference count is a digital representation of the integrated input signal difference.
One problem with this type of ADC, especially when formed as, or as part of, an integrated circuit (IC), is that the VCO transfer characteristic from input voltage to output frequency may have wide manufacturing tolerances. Thus, the frequency output for a given defined input signal may be unpredictable from IC-to-IC and/or wafer-to-wafer and hence the scale factor or gain of the ADC, i.e. the digital output for a given known input, may be unpredictable. Also the VCO transfer characteristic may be temperature dependent thus meaning that the gain or scale factor may vary in use.
Embodiments of the present invention mitigate at least some of the above mentioned problems.